Mamj. Vanzandvoort et al., THE ORIENTATION OF THE TRANSITION DIPOLE-MOMENTS OF CHLOROPHYLL-A ANDPHEOPHYTIN A IN THEIR MOLECULAR FRAME, Photochemistry and photobiology, 62(2), 1995, pp. 299-308
In this paper we describe the determination of the orientation of the
absorption and emission transition dipoles of chlorophyll a and pheoph
ytin a in their molecular frame. For this purpose we have embedded the
pigments in anhydrous nitrocellulose films with a concentration of 2
x 10(-7) mol/g. We have shown previously that under these conditions t
he pigments are in a purely monomeric state, are distributed uniformly
both before and after stretching and that no intermolecular energy tr
ansfer among the molecules takes place. Using a combination of steady-
state anisotropy experiments on unstretched films and angle-resolved f
luorescence depolarization measurements on stretched films, we obtain
the orientation of the transition dipole moments of both pigments in t
heir molecular frame and the orientational distribution function of th
e molecules relative to the stretching direction of the film. The stea
dy-state anisotropy measurements indicate that chlorophyll a has two d
istinct emission dipole moments and that excitation in the Soret-regio
n results in simultaneous excitation of two or more absorption transit
ion dipole moments. On the other hand, excitation in the Q(Y)-band inv
olves only a single dipole moment. The directions of the transition di
pole moments in the molecular frame are obtained from the angle-resolv
ed measurements. Pheophytin a also exhibits two emission dipole moment
s, but the angle between them is much smaller than that between the co
rresponding dipoles for chlorophyll a. As a consequence the dipole mom
ents contributing to the Soret-region could not be resolved and only a
n effective absorption transition dipole moment in the Soret-region is
extracted.